What a gripping story! I'm not making light of it but I was hooked, wondering what the hell would happen at the end.
I've got no fresh insight but I agree with others who say that she must have had mental health issues, and I'm wondering whether something in the situation triggered a memory for her. Perhaps a bad memory with her father? Hence her sudden jibes to you about being old and decrepit. Her taunts don't make any sense though, because if you're in your early 50s and reasonably fit, there's no way you can be described as either.
I guess we'll never know, unless she makes contact with you again online.